1. Field of the Invention
The present invention relates to the purification of admixtures of nitroaromatic compounds produced via the nitration of aromatic compounds using nitric acid in the presence of sulfuric acid.
This invention especially relates to the removal of nitro-substituted cresols from admixtures of nitroaromatic compounds comprised thereof.
2. Description of the Prior Art
Nitroaromatic compounds, in particular aromatic compounds containing two nitro groups, are conventionally prepared via a two-step process. In the first step, the aromatic reactants are contacted with a mixture of nitric and sulfuric acids. The reaction mixture is then decanted and, in a second step, the organic phase containing the mononitrated aromatic compounds is contacted with a nitric acid/sulfuric acid mixture to produce the corresponding aromatic compounds which are substituted with two nitro groups.
The mixture from the second operation, however, contains objectionable byproducts, in particular cresols, especially those which are substituted with three nitro groups (hereinafter designated the "nitrocresols").
These nitrocresol byproducts must be separated from the nitroaromatic compounds before the latter can be subjected to any further reactions. In a process for the preparation of aromatic amines from the nitroaromatic compounds indicated above, for example, the nitrocresols are known to poison the catalysts employed for the reduction of the nitro groups into amine groups.
One known technique for separating the nitrocresols from the nitroaromatic compounds is to treat said mixture with a base and with water, whether simultaneously or successively, to transform the nitrocresols into salts, and to then separate out the organic phase, which principally contains nitroaromatic compounds, from the aqueous phase which contains the salts of the nitrocresols.
The difficulty with this particular technique is in the fact that the aqueous effluent contains amounts of nitrocresol salts which are too large and exhibit a very high chemical oxygen demand. As a result, effluents containing such salts cannot be discharged untreated to waste.
It is thus necessary to chemically or biochemically treat such effluents to reduce the amount of nitrocresol salts discharged, or to incinerate same. This, of course, significantly adds to the cost of the operation.